Looking for a correct formula
Sorry to answer my own question but here is how I got through it.    

import bge
import math

scene = bge.logic.getCurrentScene()
A = scene.objects['Axis']['time']
mist = scene.world
F = math.sin(math.pi*A/2500)

if (A) &lt; (1250):
    mist.mistColor = [1.0, 1.0-F*0.1, 1.0-F*0.2]
elif ((A) &gt;= (1250)) and ((A) &lt; (2500)):
    mist.mistColor = [F*0.8+0.2, F*0.7+0.2, F*0.6+0.2]    
elif ((A) &gt; (2500)) and ((A) &lt; (3750)):
    mist.mistColor = [0.2-F*0.6, 0.2-F*0.6, 0.2-F*0.3]
elif (A) &gt;= (3750):
    mist.mistColor = [1.0 + F*0.2, 1.0+F*0.2, 1+F*0.5]